How to get reticule health?

Quackerz Gamerz

I've seen vids of people playing IV, and when they use their gun, and hover the crosshairs over an enemy, health bars appear inside the ring. I've tried to do this in my game, but so for there has been no success.

I'd really like for this to work, as it would help me out greatly knowing if I'd actually killed an enemy.

Keep in mind I'm on Xbox 360, just in case that is critical information.